1. Field of the Invention
The present invention relates to a synthetic filament yarn fabric for manufacturing an air bag.
2. Description of the Prior Art
The primary requisite of a fabric used for the manufacture of air bags is strength, with particular emphasis on the ability of the fabric to resist tearing and bursting. High strength is imparted to an airbag fabric by employing high tenacity yarns, preferably of nylon 6,6, of high mechanical quality. The linear density of the yarns, and the concomitant weaving intensity, [expressed (for example) by the total number of warp crimps in one square meter of fabric], are chosen to ensure that the strength requirement is met after weaving into selected patterns and finishing of the fabric by various means. Most commonly, a single layer, plain weave pattern is chosen because this pattern has the highest levels of crimp for a given fabric construction, i. e., yarn linear density and fabric sett. This latter attribute is described by the ends/centimeter of the warp yarn and the picks/centimeter of the fill yarn.
Whereas high strength is a necessary attribute of an air bag fabric, there are other requirements which must be additionally met. Of these additional attributes, low air permeability and good flexibility are particularly important.
In the rapid deployment of an air bag, the fabric is subjected to high operating pressures. Airbag operating pressures are orders of magnitude greater than those employed in the standard methods for the determination of the air permeability of the fabric. Whereas standard test pressures range up to 0.20 kPa, it is now common practice to generate data important to the estimation of the permeability of the fabric under practical conditions at a pressure of 2.5 kPa. It is on this "Dynamic" permeability, rather than so-called "Static" permeabilities, that selection of yarn geometry, and fabric construction and fabrication methods, are appropriately based.
Associated with good flexibility, for the purpose of being able to compactly store the airbag fabric, is the requirement of low thickness. It is also desirable that the fabric be of low weight, as a means of mitigating the impact of the deployed airbag upon the occupant.
The linear density of the filaments comprising the yarn bundle may be manipulated in order to balance the many requirements to be met by a serviceable fabric. Associated with the use of yarn of high linear densities, in excess of seven hundred (700) dtex, for example, filament densities in excess of six (6) dtex, have been employed in air bags fabrics. Representative of such a fabric is that described in Japanese Patent 07/300,054-A (Toray Industries, Inc.). Whereas combinations of linear densities of yarn and filament of the type cited above give the required strength at low weaving intensities, and whereas yarns of high mechanical quality can be provided, because the filament linear density is high, the fabrics exhibit high stiffness and high permeability.
The problem of high permeability may be addressed by the coating of the fabric, or by means of dimensional contraction of the fabric after weaving. The dimensional contraction may be accomplished through suitable post-treatment, either by dry or wet methods, that entail effecting shrinkage of the fabric. Representative of such a treated fabric is that described in EPO Patent EP 436,950 (AKZO NV.). The consequence of both approaches is that penalties are incurred in the form of higher stiffness and bulk, and increased complexity and cost of fabrication.
The problems of high stiffness and high permeability may be addressed by the device of lowering the linear density of the yarn, and of the filaments comprising the yarn. Yarn linear densities in the range of seven hundred (700) to two hundred (200) dtex have been proposed, and filament deniers from six (6) dtex to three (3) dtex, and even below three (3) dtex, have been proposed. Representative of such a fabric is that described in Japanese Patent J 04/209,846-A (Toyobo KK). The drive towards the lowering of the filament denier is greater: a) for polyester yarns than it is for polyamide yarns; and, b) as the tightness of the fabric is increased, through either the choice of the sett, or by means of post-treatment.
The linear density of the yarn is lowered at the expense of the strength of the airbag fabric, even with the concomitant increase in weaving intensity. And when the linear density of the filament comprising the yarn is lowered, the difficulty of achieving yarn of high mechanical quality is increased, it becomes more difficult to achieve high weaving efficiencies, and high mechanical quality of the woven fabric.
In view of the foregoing it is believed to be advantageous to provide a fabric for manufacturing air bags which best balances the requirements of high strength and low permeability, under test and practical conditions, while also providing a fabric which is flexible and is not bulky and heavy, and which is also inexpensive to manufacture and further process.